Thursday, September 29, 2011

• Listeria monocytogenes is Gram-positive individually or in short chains
• Non-spore forming
• Motile by means of flagella
• Aerobic
• Optimum growth temperature 30-37°C
• Can grow slowly in refrigerated foods
• There are a 6 different species within the genus but only Listeria monocytogenes is considered pathogenic for humans. Listeria is ubiquitous in the environment. It is quite hardy - resisting freezing, drying and heat. The ability to grow at temperatures as low as 3°C permits multiplication in refrigerated foods.

The organism has served as a model for the study of intracellular pathogenesis for several decades and many aspects of the pathogenic process are well understood
• Listeriae are acquired primarily through the consumption of contaminated foods including soft cheese, raw milk, deli salads, and ready-to-eat foods such as luncheon meats and frankfurters
• Although L. monocytogenes infection is usually limited to individuals that are immunocompromised, the high mortality rate associated with human listeriosis makes L. monocytogenes the leading cause of death amongst foodborne bacterial pathogens .
Approaches for Detection, Identification, and Analysis of Foodborne Pathogens
 Traditional microbiological methods for testing foods for the presence of pathogens rely on
 Growth in culture media, followed by isolation, and biochemical and serological identification.
 Traditional methods are laborious and time consuming, requiring a few days to a week or longer to complete. Rapid detection of pathogens in food is essential for ensuring the safety of food for consumers, and in the past 25 years, advances in biotechnology have resulted in the development of rapid methods that reduce the analysis time.
Major categories of rapid methods include
 Immunologic or antibody-based assays
 Genetic-based assays such as the polymerase chain reaction.
 -Generation assays under development include biosensors and DNA chips that potentially have the capability for near real-time and on-line monitoring for multiple pathogens in foods.
 Biosensor-based Detection of Food borne Pathogens
The sensitive, rapid, and specific detection of microorganisms and toxins that taint the food supply has become increasingly important as large-scale manufacture with wide distribution can threaten large populations when a contamination occurs
Listeria has been isolated from products including raw milk, cheese made from unpasteurised milk, soft cheese, ice cream, meat and poultry and fish. Ready to eat meat and poultry products are of particular risk of infection with Listeria.

Listeria monocytogenes bacteria are frequently found in the food-processing environment and can form biofilms on solid surfaces. Listeria is able to survive apparently adverse conditions on smooth surfaces; it thrives in wet, dirty conditions. The presence of Listeria species is a useful hygienic indicator in all stages of the food processing chain. Strains can spread within manufacturing plants and even establish themselves as "house flora".
In susceptible persons an infective dose can be fewer than 100 organisms.
Traditionally food and environmental samples are enriched in a broth prior to subculture into a further broth and then onto selective agar.

The initial broth incubation is at 30°C for 24 hours, the subsequent broth incubation is at 35°C for 24 hours. All broth cultures are then subcultured onto agar for a further 24 hours and subsequently identified by biochemical tests.

The traditional method is labour intensive and takes up to 5 days to give a result. There are therefore now available many commercial alternatives to generate a faster result. One of these is the use of chromogenic agars following a simple 24 hour enrichment. These agars give presumptive positive results.

As an alterative to growth on agar there are a variety of technologies that provide rapid results eg. immunoassay; molecular methods such as PCR; or techniques that reduce time to result e.g. concentration. Sometimes combinations of these techniques are used to further enhance speed to result.

Molecular methods such as PCR can eliminate the need for further identification in the event of a positive and can also provide quantitative information.

The unique challenges of rapid routine environmental monitoring for the presence of Listeria in food processing have been met by specific products.
Following isolation on selective media, identification can be carried out using agglutination, biochemical and molecular techniques.

Latex agglutination allows rapid elimination of negative samples, positives can then be checked out with more targeted tests.